The difficult work begins

The open symposium of the European Strategy for Particle Physics (ESPP) update, which drew to a close last week in Granada, Spain, was a moment for physicists to take stock of their field’s status and future. A week of high-quality presentations and focused discussions proved how far things have moved on since the previous strategy update concluded in 2013. In the past few years the LHC has proved the existence of the Higgs boson and so far suggested that there are no new particles beyond the SM at the electroweak scale. Spectacular progress has been made with neutrinos, dark-matter searches, flavour and electroweak physics, and gravitational-wave astronomy is beginning to take off. The deepest puzzles of the standard models of particle physics and cosmology remain at large, however, and large colliders are one of the best tools to address them.

Recommendations from the ESPP are due early next year. Dominating discussions at the open symposium last week was which project should succeed the LHC after its operations cease in the 2030s. The decision has significant consequences for the next generation of particle physicists, not just in Europe but internationally. Perspectives from Asia and the Americas, in addition to national views and inputs from the astroparticle– and nuclear-physics communities, brought into sharp focus the global nature of modern high-energy physics and the need for greater coordination at all levels.

The 130 or so talks and discussion sessions in Granada revealed a community united in its desire for a post-LHC collider, but less so in its choice of that collider’s form. Enormous efforts have gone into weighing up the physics reach of the various projects under study, a task complicated by the complexity of future accelerator technologies, detectors and analyses. Stimulating some heated exchanges, the ESPP saw the International Linear Collider (ILC) in Japan, a Compact Linear Collider (CLIC) or future circular electron–positron collider (FCC-ee) at CERN and a Circular Electron Positron Collider in China (CEPC) pitted against each other and against expectations from the high-luminosity LHC in terms of their potential in key areas such as Higgs physics.

Summary sessions

Summing up the situation for beyond-SM (BSM) physics, Gian Giudice of CERN said that the remaining BSM-physics space is “huge”, and pointed to four big questions for colliders: to what extent can we tell whether the Higgs is fundamental or composite? Are there new interactions or new particles around or above the electroweak scale? What cases of thermal relic WIMPs are still unprobed and can be fully covered by future collider searches? And to what extent can current or future accelerators probe feebly interacting sectors?

Neutrinos, the least well known of all the SM particles, were the subject of numerous presentations. The ESPP audience was reminded that neutrino masses, as established by neutrino oscillations, are the first particle-physics evidence for BSM phenomena. A vibrant programme is under way to fully measure the neutrino mixing matrix and in particular the neutrino mass ordering and CP violation phase. Other experiments are probing the neutrino’s absolute mass scale and testing whether they are of a Dirac or Majorana nature. Along with gravitational waves, neutrinos play a powerful role in multimesseneger astronomy.

Around a fifth of the 160 input documents to the ESPP were linked to flavour physics, covering topics such as lepton-flavour universality, electric-dipole moments and heavy-flavour studies.

Flavour physics is crucial for BSM searches since it is potentially sensitive to effects at scales as high as 10⁵ TeV, said Antonio Zoccoli of INFN in his summary. There is also much complementarity between low-energy physics, the high-energy frontier and searches for feebly interacting particles, he said. Oddities in b-decays seen by the LHCb collaboration are of particular interest. “Flavour is a major legacy of LHC,” Zoccoli concluded. “Charged hadron particle-ID should be mandatory for a full physics programme at future colliders.”

Summarising ESPP sessions on dark-matter and dark-sector physics, Shoji Asai of the University of Tokyo drew attention to a shift in sociology that is taking place. In the old view, dark-matter solutions arose as a byproduct of “top-down” approaches (such as supersymmetry) to solve the SM’s problems. The “new sociology” holds that dark matter needs an explanation of its own, and it’s to be considered a bonus if such a solution also elucidates important issues such as the strong-CP problem or baryogenesis. Among the “big questions” identified in this sector at the ESPP update were: What are the main differences between light hidden-sector dark matter and WIMPs? How broad is the parameter space for the QCD axion? How do we compare the results of different experiments in a more model-independent way? And how will direct and indirect dark-matter detection experiments inform/guide accelerator searches and vice versa? Asai said that consensus has emerged on the need for more coordination and support between accelerator-based direct detection and indirect detection dark-sector searches, as exemplified by the new European Center for AstroParticle Theory.

In summarising interests in the strong sector, Jorgen D’Hondt of Vrije Universiteit Brussel listed the many dedicated experiments in this area and the open questions identified at the ESPP symposium: “What are the experimental and theoretical prerequisites to reach an adequate precision of perturbative and non-perturbative QCD predictions at the highest energies? What can be learned from beams-on-target experiments at current and potential future accelerators? How to probe the quark–gluon plasma equation of state and to establish whether there is a first-order phase transition at high baryon density? What is known about the make-up of the proton (mass, radius, spin, etc) and how to extract it? And what is the role of strong interactions at very low and very high (up to astrophysical) energies?”

Electroweak sparks

Of all the scientific themes of the week, electroweak physics generated the most lively discussions, especially concerning how well the Higgs boson’s couplings to fermions, gauge bosons and to itself can be probed at current and future colliders. Summary speaker Beate Heinemann of DESY cautioned that such quantitative estimates should be treated with a degree of flexibility at this time, though a few things stand out: one is the impressive estimated performance from the HL-LHC in the next 15 or so years; another is that a long-term physics programme based on successive machines in a 100 km-circumference tunnel offers the largest overall physics reach on the Higgs boson and other key parameters. The long timescales required to master the technology for the next hadron collider were well noted. There is broad agreement that the next major collider after the LHC should collide electrons and positrons to fully explore the Higgs boson and make precision measurements of other electroweak parameters that are sensitive to phenomena at higher energy scales. Whether that machine is circular or linear, and built in Asia or Europe, are the billion-dollar questions facing the community now.

The closer involvement of particle physics with astroparticle physics, in particular following the discovery of gravitational waves, was the running theme of the open symposium. It was argued that, in terms of technology, next-generation gravitational-wave detectors such as the Einstein Telescope are essentially “accelerators without beams” and that CERN’s expertise in vacuum and cryogenic technologies (a result of the lab’s continual pursuit and execution of big-collider projects) would help to make such facilities a reality.

The closing discussion of the symposium offered a final hour for physicists to air their views, many of which were met with applause. Proponents of circular machines highlighted the high flexibility and exploratory potential of projects such as FCC-ee, pointing out that it would serve as an electroweak as well as a Higgs factory. Linear-minded participants cited factors such as the extendable nature of linacs, and the independence of their tunnels from a subsequent hadron collider. For others, the priority for CERN should be to enter negotiations as soon as possible for a 100 km tunnel in the Geneva region, buying time to decide which physics option should be installed. Warm applause followed a remark that CERN decides for itself what its next project should be, without relying on other labs. But there were reminders from others that high-energy physics is an international field and that, in times of scarce resources, all options should be considered.

The high-energy physics community has risen to the occasion of the ESPP update. New thinking, from basic theory to instrumentation, computing, analysis and global organisation, is clearly required to sustain the recent rate of progress. Now that the open symposium is over, the European Strategy Group (ESG) will start to prepare a briefing book. Further input can be submitted to the strategy secretariat during the next months, and at a special session organised by the European Committee for Future Accelerators on 14 July 2019 during the European Physical Society Conference on High Energy Physics in Ghent, Belgium. An ESG drafting session will take place on 20–24 January 2020 in Bad Honnef, Germany, and the update of the ESPP is due to be completed and approved by the CERN Council in May 2020.